The steering systems of highway vehicles and the like are designed primarily for driver control. In these systems, the steering force required on the steering wheel and the ratio between steering wheel movement and movement of the steerable ground wheels depend upon the characteristics of the particular vehicle and the conditions under which it will usually be operated. A wide variety of extraneous forces can act on a vehicle steering system and spurious steering inputs caused by these forces must be dealt with satisfactorily in order to provide stable and controllable steering of a vehicle. As vehicle speed increases, the effects of any spurious steering inputs are magnified, making it necessary for the driver to exercise more precise and careful driving control.
The transmission of these spurious inputs between the steerable wheels and the steering wheel causes the interconnecting components of the steering system to repeatedly oscillate between states of tension and compression. Such oscillations cause wear and slack in ball joints and other connections and have long been considered a primary source of stress fatigue which can lead to premature failure of various steering system components. Mechanical slack due to worn parts can also be a cause of steering system oscillations and vehicle wandering that require constant corrections and therefore contribute to driver fatigue.
The ideal driving situation is therefore one where the steering system inherently causes the vehicle to travel in an unswerving straight line unless the driver intentionally turns the vehicle in another direction. Thus, the ideal steering system would require relatively little attention from the driver as the vehicle progresses along a straight line path down the roadway. From a steering standpoint, the vehicle should not respond to anything but the driver's steering commands and these must be of sufficient magnitude to overcome a significant resistance to turning away from center. In the absence of a steering input by the driver, the vehicle should literally do nothing but progress straight ahead.
Vehicles with steering systems having positive caster generally track relatively straight ahead and resist steering inputs away from center, including those of the driver, provided that the roadway they are travelling on is smooth and is not slanted or crowned. Such positive caster is provided by positive caster offset, which is also known as mechanical trail. Caster offset is the distance from the ground intersection point of a pivot line drawn through the pivot axis of a steerable wheel to a contact point at the center of the area over which the wheel contacts the ground. The pivot axis of a steerable wheel of a motor vehicle is usually provided by a "king pin". Because the contact point of a steerable wheel with positive caster trails the pivot line point of the wheel, side forces cause the wheel to turn in the direction that the force is being applied. A good example of this is the way in which the castered wheels on the front of a shopping cart are easily turned in the direction of applied force.
The adverse effects described below are some of the negative aspects of achieving steering stability with positive caster offset. Because of the side force applied by gravitational pull on a slanted or crowned highway, positive caster offset causes a motor vehicle to freely turn to the low side, creating a steering wheel pull that requires counteractive steering input from the driver to keep the vehicle from leaving the highway. The amount of driving fatigue that is directly caused by positive caster offset under these conditions may be appreciated by considering the many millions of miles driven by truck drivers and other motorists each day on crowned or slanted highways.
Another fatiguing driving condition that may be encountered by a motorist is that of controlling a crosswind steering input. The amount of adverse steering input caused by crosswinds is directly related to the amount of positive caster offset, which is a classic example of having to balance a benefit with a detriment. The small amount of stability gained from castering the steerable wheels on a non-windy day may be paid for many times over when driving in a crosswind because of the destabilizing effect of the crosswind when combined with positive caster offset. Positive caster offset also allows steering inputs from rutted and other imperfect roadway surfaces to steer back against the driver and thereby cause road wander, which is a universal driving complaint, particularly by driver's of heavy vehicles such as trucks and motor homes.
For the lack of a more advanced method, steerable wheel castering has been accepted by the industry as a low-cost method of achieving steerable wheel returnability. Thus, large, heavy over-the-road vehicles are presently provided with generous amounts of positive caster. Not much thought has been given to the self-defeating side effects of steerable wheel castering. Instead, the lack of directional stability is blamed on the size and weight of the vehicle.
As the size and weight of over-the-road vehicles increases, the need for directional stability becomes more important. Learning to drive a heavy vehicle means learning to control the back steer caused by the adverse side effects of steerable wheel castering. The failure of the industry to recognize the critical need to provide directional stability by replacing steerable wheel castering with another method of achieving steerable wheel returnability may go down in history as one of the longest enduring heavy vehicle design oversights.
The lack of directional stability is fundamentally the reason that heavy vehicle driving is much more stressful than it otherwise needs to be. Keeping a heavy vehicle, that is lacking in directional stability, tracking straight and under control for extended periods of time is a major cause of driving fatigue and related accident potential. The failure of numerous driver fatigue and alertness studies to consider the contribution made by "driving" fatigue in the overall evaluation is indicative of the wide-spread failure of the industry to recognize the lack of directional stability as the major cause of driver fatigue resulting from driving fatigue. Accordingly, a dramatic reduction in driver fatigue may be made by making heavy over-the-road vehicles directionally stable and thereby significantly reducing driving fatigue.
The term "directional stability" does not legitimately apply to the current production of heavy vehicles because they are, in fact, not directionally stable. The lack of heavy vehicle directional stability is not the fault of the steering gear. The purpose of past improvements in the art of steering gears and other steering components has been to make it easier for the driver to control the unstable behavior of castered steerable wheels. Irrespective of such refinements in the steering gear and related components, when the steerable wheels are allowed to caster, the driver will still have to make the same excessive number of steering corrections to control road wander, slanted road steering wheel pull, and down wind steerable wheel castering.
Thus, a highly important consideration that has long been overlooked by the industry is that steerable wheel castering is directly responsible for road wander, crowned road steering wheel pull and cross wind steering problems. Keeping an unstable heavy vehicle tracking straight and under control currently requires an inordinate amount of driver steering corrections to counteract the adverse side effects of castered wheels. The repetitive task of making thousands of precise steering corrections mile after mile weighs heavily on a driver's physical and mental well-being, and may result in extreme driving fatigue. Thus, vehicle directional stability especially for heavy vehicles can only be achieved by stabilizing the on-center behavior of the steerable wheels with a more suitable method than the traditional steerable wheel castering used on all current production vehicles.